Polymer substrates with a large number of microfibers on a surface have a wide variety of potential applications. Such microstructured polymer films may be applied to a surface in order to decrease the gloss of the surface. Other surfaces which may benefit from the application of materials having increased surface area due to the presence of a large number of microfibers include carrier webs for use with adhesive tapes. Polymer surfaces covered with a plurality of microfibers also typically have a soft or cloth-like feel and can provide a low friction surface. Polymer sheet materials with smooth planar surfaces are often treated to provide fibers or fiberlike features protruding from at least one major surface. Alteration of a surface in this manner can produce a number of effects, e.g., a decorative appearance, the dispersion of incident light, increased wicking of fluids and/or a low friction surface.
A variety of methods for producing polymer films having a surface with a suede-like feel are known. For example, one of the oldest methods of achieving this effect is called flocking. This involves attaching one end of chopped fibers to a planar surface. Various methods have been used to position the fibers perpendicular to the planar surface (e.g., U.S. Pat. No. 3,973,059 or U.S. Pat. No. 5,403,884). Woven textiles are often passed through a napping machine which pulls loops of small strands from the woven article. The small pulled fibers may break or simply form a loop. The overall napping process typically imparts a soft feel to the napped surface of the article. Another approach which has been used to alter the surface of materials such as leather is to abrade the surface with abrasives such as sand paper. Processes of this type are used to make suede leather. A suede-like feel has been imparted to the surface of polymer foam materials by heat skiving the surface so that the thin sidewalls of the ruptured foam cells provide a soft feel to the treated surface (see, e.g., U.S. Pat. Nos. 3,814,644 and 3,607,493). Yet another method, such as disclosed in U.S. Pat. No. 5,403,478, involves bonding a non-woven sheet onto a plastic film. A suede-like feel has also been achieved by the extrusion of fibers onto a thermoplastic polymer film and heat bonding the fibers to the film (see, e.g., U.S. Pat. Nos. 3,152,002, 4,025,678 and 5,403,884).
Several patents (e.g. U.S. Pat. Nos. 5,116,563; 5,230,851; and 5,326,415) disclose a substrate having a plurality of tapered prongs on a surface. The prongs are formed by depositing islands of heated, thermally sensitive material (e.g., a thermoplastic material) onto the moving substrate surface such that a velocity differential exists between the depositing thermally sensitive material and the underlying substrate surface. The tapered prongs typically have a base diameter of about 700–1300 microns and heights of about 500–2000 microns. Other methods of forming tapered thermoplastic projections on an underlying sheet have also been reported. U.S. Pat. No. 3,027,595 discloses the formation of an artificial velvet fabric having a plurality of pile-like projections. The projections are formed by contacting a thermoplastic sheet with the heated surface of a drum having a multiplicity of closely spaced conical depressions in its surface. The exemplary pile-like projections disclosed have a base diameter of about 150 microns and a length of about 3000 microns (3 mm). U.S. Pat. No. 5,407,735 discloses a napped polyester fabric having sheath-core polyester fibers with a tapered tip. The fibers typically have a fineness in the range of 2 to 6 deniers and pile lengths of about 3 mm.
In order for the articles containing microstructured polymer materials to realize their full potential, versatile, inexpensive methods of fabricating such polymer materials must be available. Current methods typically only permit the generation of polymer substrates with limited types of microstructure configurations. A need, therefore, continues to exist for improved methods of producing polymer substrates having a surface with a napped texture. Such methods would preferably permit the production of polymer substrates with a defined microscopic pattern. Optimally, the method would also permit the introduction of macroscopic structural features (e.g., via embossing) and/or would allow the choice of generating a microscopic pattern on either all or a portion of the surface.